Sri Gurubhyo namah Sri Parama GurubhyO namah
Sri Parameshthi GurubhyO namah

82 nd sloka in Chapter 2 of Varivasya Rahasya by Sri Bhaskaramakhin gives the meaning of Nigarbha Artha. The meaning given by Yogini Hrudaya as well in this sloka is almost same.
Oneness among Supreme Siva, Guru and Atman is achieved only by relentless practice. The experience of oneness of the Atman with Siva cannot be explained because it is an experience. This is the Nigarbha Artha.
Let us see what Yogini Hrudaya say.
This explains Siva, Guru and the disciple or worshipper or an adept. Supreme Siva is Parabrahma, Guru is the one who has felt, seen this Parabrahma in his Atma as well who knows the Agamas. Disciple is one who has the experience of oneness with the supreme siva.
Guru though is in the form of an ordinary human being; he is obviously none other than Siva himself.
The disciple get relieved of all his sins and gets the knowledge of Brahman by the grace of GURU having served him and having understood the Parabrahma in Siva.
Nigarbha is that where Guru, Sishya and Parabrahma is/are seen without any difference among these three.

7 meanings of Sri vidya panchadasi Mantra links:

1st meaning – Gayathri

2nd meaning – Bhavartha

3rd meaning – Sampradayartha

4th meaning – Nigarbha artha

5th meaning – Kaulikartha

6th meaning – Sarva Rahasyartha

7th meaning – Mahatatvartha

LOKA SAMASTA SUKHINO BHAVANTU SAMASTA SANMANGALANI BHAVANTU